Histone deacetylase (HDAC) proteins are key enzymes involved in epigenetic regulation of gene expression. Specifically, HDAC-mediated deacetylation of acetyllysine residues on nucleosomal histones leads to tight binding to genomic DNA, which affects accessibility and transcription.1-2 In addition, HDAC proteins influence protein-protein interaction, protein-DNA interaction, protein localization, and protein stability through deacetylation of non-histone substrates.3-4 The eighteen human HDAC proteins are divided into four classes according to their homology with yeast proteins, size, cellular localization, and number of catalytic active sites.5 Class III (SIRT1-7) HDAC proteins are NAD+-dependent. Classes I (HDAC1, 2, 3 and 8), II (HDAC4, 5, 6, 7, 9, and 10), and IV (HDAC11) HDAC proteins are metal-dependent, and are the focus of this work.5 
HDAC proteins regulate the expression of several cancer-related proteins involved in cell signaling, transcription, and tumor suppression through the deacetylation of nucleosomal histone proteins.6-7 Overexpression of HDAC proteins results in unregulated transcription and aberrant protein activity and function, which is linked to several diseases, including cancer.7 For example, HDAC1 was overexpressed in lung,8 breast,9 and colon cancers.10 HDAC2 was overexpressed in colorectal cancer.11 HDAC8 was highly expressed in neuroblastoma patients, leading to cancer progression and poor survival rates.12 In addition, selective inhibition of HDAC8 induced apoptosis in leukemia and T-cell lymphoma cell lines.13 Class II HDAC6 was overexpressed in oral squamous cell carcinoma and ovarian cancer.14-15 Overexpression of both HDAC6 and HDAC8 was linked to breast cancer metastasis and invasion.16 
Due to their key role in cancer, several anti-cancer agents targeting HDAC proteins have been developed.17 HDAC inhibitors promoted apoptosis and reduced proliferation and migration through their effect on both histone and non-histone substrates.17-19 Several HDAC inhibitors have been approved by the FDA for treatment of cancer, and several others are in clinical trials.20 SAHA (suberoylamide hydroxamic acid, Vorinostat, Zolinza™), and Belinostat (PXD101, Belodaq™) are FDA-approved for treatment of T-cell lymphoma (FIG. 1),20-22 while Panobinostat (LBH-589, Farydak™) was approved for treatment of multiple myeloma (FIG. 1).23 SAHA is a nonselective inhibitor that targets most of the eleven metal-dependent HDAC isoforms.24 The nonselectivity of the FDA-approved drugs, including SAHA, might explain the side effects observed in the clinic, such as cardiac arrhythmia and thrombocytopenia.25-26 Moreover, the use of SAHA as a chemical tool to study the role of specific HDAC isoforms in cancer cell biology is limited due to its nonselectivity.
To overcome the limitations of nonselective drugs, several isoform selective HDAC inhibitors have been developed, with some in clinical trials. As illustrative examples, entinostat (MS-275, FIG. 1) is selective for HDAC1, 2, and 3,24, 27 whereas tubastatin (FIG. 1) is HDAC6-selective.28-29 Recently, several dual HDAC6/8 selective inhibitors have been reported, including BRD-73954 and valpropylhydroxamic acid (FIG. 1).30-31 HDAC inhibitors that target one or two HDAC isoforms will be valuable for development of new drugs with minimal side effects.32-35 In addition, recent reports suggested that inhibition of two HDAC isoforms is desirable by maintaining synergistic therapeutic effects in various cancers.30, 36-37 Related to this work, dual inhibition of HDAC6 and HDAC8 might have potential application in breast cancer angiogenesis and metastasis.13, 30 Moreover, selective HDAC inhibitors will be useful as chemical tools to study cancer-related HDAC cell biology.
To understand the structural requirements of HDAC inhibitors, SAHA analogs substituted have been synthesized in the linker region at carbon 2 (C2), carbon 3 (C3), or carbon 6 (C6) (FIG. 1).38-40 C2-hexyl SAHA (FIG. 1) showed HDAC6/8 dual selectivity over HDAC1, 2, and 3, with 0.6 and 2.0 μM potency against HDAC6 and HDAC8, respectively.41 Some of the C3-modified SAHA analogs displayed preference for HDAC6 over HDAC1 and 3,39 while some of the C6-modified SAHA analogs inhibited HDAC1 and 6 over HDAC3.40 In addition, SAHA analogs modified at the hydroxamic acid moiety had a preference for HDAC1.42 
Accordingly, there is a need for additional SAHA analogs with improved histone deacetylase inhibition.